1. Field of the Invention
The present invention relates to an ultrafine nickel powder which may be used for an internal electrode of a laminated ceramic capacitor, an electrode of a secondary cell, a fuel cell or the like.
2. Description of the Related Art
Much attention has been paid to an ultrafine nickel powder as material to make up electrodes of various parts of electronic devices. Such electrodes include an internal electrode of a laminated ceramic capacitor, a porous electrode of a hydrogen nickel secondary cell, and a hollow porous electrode of a fuel cell for taking out electric energy by electrochemically carrying out oxidization reaction of fuel.
The following will describe the ultrafine nickel powder, mainly giving an internal electrode of the laminated ceramic capacitor as an example.
The laminated ceramic capacitor is a member obtained by laminating dielectric ceramics, such as titanium oxide, barium titanate or complex perovskite, and metal inner electrodes alternately in layers, compressing and sintering it to be united. Recently, the market for the laminated ceramic capacitor for parts of electronic devices has grown rapidly. With the advance of the performance of electrical devices, the laminated ceramic capacitor has been promoted to be downsized and have larger capacitance. Therefore, the inner electrode has been made thinner. In the prior art, palladium was used for the inner electrode of the laminated ceramic capacitor. Recently, however, use of nickel has been increasing because of low price and high reliability.
Japanese Patent Application Laid-Open No. Hei. 1-136910 discloses an invention for producing a nickel powder having a purity of 99% or more and a particle size of 0.1 to 0.3 μm in a wet process. However, this publication dose not include the description that paste is actually produced from the particles and the paste is used for an electrode of electrical parts. The inventors have investigated the nickel powder, and then found out that the inner electrode made by using the paste of the nickel powder produced in the wet process as in this prior art and laminating it in layers is large in volume change when sintered, so as to easily cause delamination and/or cracks. In the wet process, the crystals in the particles of the ultrafine nickel powder do not grow up to large sizes, and become a cluster of primary particles because the temperature at which the ultrafine nickel powder is produced is relatively low, for example, lower than 100° C. Thus, the internal electrode is easily sintered excessively or is large in volume change when sintered.
Japanese Patent Application Laid-Open No. Sho. 64-80007 discloses paste for electrodes of a porcelain condenser using a nickel powder having an average particle size of 1.0 μm and a purity of 99.9%. In order to prevent generation of cracks and/or delamination when this electrode paste is sintered, addition of carbide particles into this paste is described. However, this publication never describes any influence of the characteristics of the nickel particles themselves on generation of the cracks and/or delamination.
In the production of any laminated ceramic capacitor, it is important techniques to prevent generation of cracks and/or delamination when sintered, as well as to make thinner layers, miniaturize the size and achieve high capacitance of the inner electrode. Thus, an ultrafine nickel powder has been desired which make it possible to restrain generation of cracks and/or delamination and produce a material for an electrode having a low resistivity.